6.6 earthquake rocks Southern California, felt by much of southwest

RIDGECREST, California — The magnitude 6.4 earthquake that struck an area of California between Los Angeles and Las Vegas on Thursday morning was the strongest quake to hit the region in 20 years.

The rolling temblor sent dozens of aftershocks through the Fourth of July holiday and researchers say they could last for a week. A California official says some injuries and two house fires have been reported in a town near the epicenter.

The United States Geological Survey says on its website that their forecast over the next week includes a 9% chance of one or more aftershocks larger than the Thursday earthquake's magnitude 6.4. It is likely that there will be smaller earthquakes over the next week, with 50 to 700 magnitude 3 or higher aftershocks.

The earthquake happened shortly after 10:30 a.m. near Searles Valley, California, about 125 miles northeast of Los Angeles and 125 miles west of Las Vegas. The 28,000 residents of Ridgecrest make up the biggest population concentration closest to the epicenter. The quake had a depth of 5.4 miles.

Kern County Fire Chief David Witt told The Associated Press that emergency crews were dealing with small vegetation fires, cracks on some roads and gas leaks that were reported after the quake in the Mojave Desert.

He also said 15 patients were evacuated from the Ridgecrest Regional Hospital as a precaution and out of concern for aftershocks.

The U.S.G.S. says magnitude 3 and above are large enough to be felt near the epicenter, and that the number of aftershocks will drop off over time. However, a large aftershock can increase the numbers again, temporarily.

The survey tweeted that at around 1:15 p.m. local time, nearly three hours after the mainshock, there were more than 100 aftershocks at least magnitude 2.5. It also said that hours later there had been over 22 that were strong enough to feel.

Veteran seismologist Lucy Jones says the earthquake was the strongest to hit Southern California in two decades. She says the previous large quake was a 7.1 that struck in the area on October 16, 1999.

Jones told reporters at the California Institute of Technology in Pasadena that the 6.4 quake centered in the desert near Ridgecrest was preceded by a magnitude 4.3 temblor about a half hour earlier.

California Gov. Gavin Newsom declared a state of emergency for Kern County, meaning the state will help the county and municipalities in it with emergency aid and recovery efforts.